Smell and Memory

A new paper by scientists at the Weizmann Institute documents the primal connection between whiffs of smell and episodic memory. This nasal nostalgia is mediated by the hippocampus, the manufacturer of long-term memory in the brain. Here's the abstract:

Authors, poets, and scientists have been fascinated by the strength of childhood olfactory memories. Indeed, in long-term memory, the first odor-to-object association was stronger than subsequent associations of the same odor with other objects. Here we tested the hypothesis that first odor associations enjoy a privileged brain representation. Because emotion impacts memory, we further asked whether the pleasantness of an odor would influence such a representation. On day 1, we associated the same visual objects initially with one, and subsequently with a second, set of pleasant and unpleasant olfactory and auditory stimuli. One week later, we presented the same visual objects and tested odor-associative memory concurrent with functional magnetic resonance brain imaging. We found that the power (% remembered) of early associations was enhanced when they were unpleasant, regardless of whether they were olfactory or auditory. Brain imaging, however, revealed a unique hippocampal activation for early olfactory but not auditory associations, regardless of whether they were pleasant or unpleasant. Activity within the hippocampus on day 1 predicted the olfactory but not auditory associations that would be remembered one week later. These findings confirmed the hypothesis of a privileged brain representation for first olfactory associations.

As the scientists note, artists have long described the powerful linkage of smell and the past. Here's Marcel Proust, explaining the madeleine:

"When from a long distant past nothing subsists, after the people are dead, after the things are broken and scattered, taste and smell alone, more fragile but enduring, more unsubstantial, more persistent, more faithful, remain poised a long time, like souls, remembering, waiting, hoping, amid the ruins of all the rest; and bear unflinchingly, in the tiny and almost impalpable drop of their essence, the vast structure of recollection."

As I noted in Proust Was A Neuroscientist, these ornate subclauses contain some prophetic insights into how our brain works. In 1911, the year Proust began writing his novel, anatomists had no idea how our senses connected inside the skull - the brain was three pounds of mysterious mush. One of Proust's hypotheses, however, was that our senses of smell and taste bear a unique burden of memory. That's why he makes it clear that just looking at the seashell shaped cookie, which he'd glimpsed countless times in patisserie windows, brought back nothing; Combray remained lost. In fact, Proust even goes so far as to blame his sense of sight for obscuring his childhood memories in the first place. "Perhaps because I had so often seen such madelines without tasting them," Proust writes, "that their image had disassociated itself from those Combray days." Luckily for literature, Proust decided to put the cookie in his mouth. As he writes, it was "by taste and smell alone" that his childhood memories came flooding back.

Why is smell so sentimental? One possibility, which is supported by this recent experiment, is that the olfactory cortex has a direct neural link to the hippocampus. In contrast, all of our other senses (sight, touch and hearing) are first processed somewhere else - they go to the thalamus - and only then make their way to our memory center. This helps explain why we're so dependent on metaphors to describe taste and smell. We always describe foods by comparing them to something else, which we've tasted before. ("These madeleines taste just like my grandmother's madeleines!" Or: "These madeleines taste like the inside of a lemon poppy seed cake!") In contrast, we have a rich language of adjectives to describe what we see and hear, which allows us to define the sensory stimulus in lucid detail. As a result, we don't have to lean so heavily on simile and comparison.

It's also worth noting, of course, that the data doesn't quite support the strong version of the Proustian hypothesis. While olfactory associations enjoy a "privileged brain representation," that hippocampal link is less important than the unpleasantness of the smell, which is much better at predicting whether or not we'll remember the memory a few days later. This is the bleak truth of the brain: it clings to what we don't like.

For more, check out the work of Rachel Herz. And thanks to Jason Synder for the tip!

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Does anyone manage to create a vocabulary of smell sensations or a philosophy (phenomenology) of odours?

Because we have names for color, tastes... describing their respective "qualia" but there is no clear and cut delimitation in the sphere of touch and smell.

Am reading your first book and loving it - though I am not yet on Proust chapter.
Regarding olfactory memory being one of the strongest: could it be that it's because olfaction is the only sense that is not situated in the cortex but beneath it (suggesting *very* early evolutionary origins)? This suggests a very privileged, if you will, access to memory, a tool that even the simplest organisms seesm to possess, to some extent, in order to survive. Harder to navigate the world without smell than without vision or hearing.
As a biology student at McGill in the late 70s, shortly after I fell in love with all things neuro, I had a pt job feeding large snails for a neurobiologist. One day I mused, "Isn't it funny that smell seems to be the strongest memory?" And he said, surprised, "Oh, is it? Wonder why." And we came up with the above reason, which I've held onto for 3 decades, failing to find a better explanation.

Will you elaborate on the supposition that the olfactory anatomy explains the need for metaphor-like expressions to describe odors. We also use metaphors to describe visual sensations, as 'that tree prays to heaven'.

By Anthony Sebastian (not verified) on 10 Nov 2009 #permalink

nobody wondered why two places in brain has regenerative powers are subgranular layer in hippocampus and subventricular neurons which migrate to olfactory glomeruli?!?!?!?

My godfather worked as a Labourer at the Singapore Habour when I was barely 4. When my mother took me to the market at the Habour,we would pass him by at the "shophouse" warehouse where he'd worked. His job was to humanly haul Gunny Bags of Nutmegs on and off lorries- yes, without a forklift.

In the years after, whenever I scent Nutmeg,an instant mental visual of that scene would unfailingly flash across my mind, and stay for a bit, trailing a memory recall that seemingly has been forgotten.

It has been 36 years since, the scent of nutmeg still has that visible power over me.

Ah, but them scents.

Enjoy enjoy enjoy!

When I was looking into a story (still unwritten) about spatial cognition, I encountered a (gentle) debate about what was more elemental and foundational to our (and our evolutionary ancestors') powers of abstraction and association. One is that the power to navigate and understand space was (and remains) a (the?) primary way to remember the world and organize experience, and thus forms a neural and abstractive foundation atop which other powers of abstraction are built. (Someday I'll write that article.) Another argument offers: Nay nay -- it's smell that predates that, as even immobile creatures need to rapidly identify and act on associations tied to smell.

All roads lead to the hippocampus. But through the nose, or otherwise? The debate continues ...

I do not remember where I heard or read it, I am not even sure if the story is historically viable at all (there are also all kinds of logistic problems with this scenario), but there it is:

Some centuries ago when the hunters would leave the settlement and go into the woods for several days or weeks at a time,they would always have several vials of distinctively smelling substances with them. Every time they would come across something significant they wanted to remember, they would open one of the vials and take a sniff.

Afterwards, if they wanted to recall the event/place at any time - all they had to do is to take a sniff from the same vial and the memory would come back in great detail.

There's a much less strained explanation for why we describe smells through simile: our perceptions of vision and sound are low-dimensionality spaces. Most of our visual words describe positions in three-dimensional color space or frequency patterns in two-dimensional projected space. Likewise our aural words describe high-frequency distribution and amplitude and patterns of low-frequency.

Low-dimensionality spaces lend themselves to description because you can formulate common "building blocks" for discussion.

Whereas smell is a very high-dimensionality space: the profile of a smell is a histogram across hundreds or thousands of unique chemical receptors. That makes building block terms like colors much less useful, because you'd need hundreds instead of a handful. So we describe smells in the only way that makes sense: by citing known samples whose profiles are similar.

By pottedplant (not verified) on 14 Nov 2009 #permalink

I think smells play a big part in early childhood memories because young children lack the expressive language to describe experiences and store and then recall them primarily as sensory experiences. As we acquire language, sensory memories like smell get crowded out of our recollections of those later-in-life experiences.

The information in this post reminds me of the beginning of Emotional Intelligence by Daniel Goleman (…) As far as I recall, this has a lot to do with evolution, and something called the rhinoceptron.

I've long harboured the belief that people with sensitive noses tend to be more emotionally sensitive as well.